The present invention relates to a dynamic strain measurement circuit, and more particularly, to a circuit design for maintaining a constant output from the circuit despite changes in lead-wire resistance.
Strain gages are typically used to measure dynamic stress on an object. One typical field in which a strain gage is used is in the monitoring of dynamic stress on turbine and compressor blades in turbo jet engine testing. Usually these strain gages are single active arm strain gages. The lead wires connected to these strain gages are typically of a small diameter with a high resistance.
Changes in temperature can effect changes in the effective resistance of the lead wires. This change is significant when compared to the change in gage resistance from the strain. Because there is no direct access to the gage itself, it is not possible to use three and four wire resistance measurement techniques to detect or compensate for these resistance fluctuations. Furthermore, it is not possible to connect the signal conditioner guard shield to a source of common mode voltage.
It has been known to provide one of two different types of signal conditioners: a) a conditioner having a constant voltage excitation supply; and b) a conditioner having a constant current excitation supply. The constant voltage type conditioner has excellent common-mode rejection of unwanted signals. However, such a conditioner is sensitive to changes to the resistance of the lead wires going to the strain gage. Constant current conditioners are immune to such lead wire resistance changes However, because of their inherently unbalanced output impedances, such constant current conditioners typically have very poor common-mode rejection. In addition, because of their wide active band width, constant current conditioners are typically noisier than constant voltage conditioners.
Thus, the problem exists of providing common mode rejection and a minimization of the impact of changes in lead wire resistance.